TY - JOUR T1 - Theoretical kinetics of sequential metabolism in vitro. Study of the formation of 16 alpha-hydroxyandrostenedione from testosterone by purified rat P450 2C11. JF - Drug Metabolism and Disposition JO - Drug Metab Dispos SP - 584 LP - 591 VL - 22 IS - 4 AU - K Sugiyama AU - K Nagata AU - J R Gillette AU - J F Darbyshire Y1 - 1994/07/01 UR - http://dmd.aspetjournals.org/content/22/4/584.abstract N2 - P450 2C11 from rat liver is known to metabolize testosterone to 2 alpha-, 16 alpha-, and 6 beta-hydroxytestosterone, and to androstenedione and 16 alpha-hydroxyandrostenedione. Because Waxman (J. Biol. Chem. 259, 15481-15490) has reported that the enzyme converts androstenedione to 16 alpha-hydroxyandrostenedione, it seemed likely that the metabolite was formed from testosterone by way of androstenedione. Indeed, we have found that P450 2C11 does not convert 16 alpha-hydroxytestosterone to 16 alpha-hydroxyandrostenedione to any significant extent and, therefore, that the metabolite is formed from testosterone almost solely by way of androstenedione. To determine whether some of the 16 alpha-hydroxyandrostenedione might be formed directly from the androstenedione-enzyme complex, we developed an approach by which it is possible to calculate the amount of the androstenedione, released into the medium, relative to the amount of the androstenedione-enzyme complex that is converted directly to 16 alpha-hydroxyandrostenedione under initial conditions when the concentration of released androstenedione will be negligible. The approach uses two factors: factor A is the androstenedione/(androstenedione + 16 alpha-hydroxyandrostenedione) present at the end of the incubation, and factor B corrects for the amount of released androstenedione that recombines with the enzyme and is converted to 16 alpha-hydroxyandrostenedione. Although the values of both factors A and B will vary with the concentrations of testosterone and preformed androstenedione present in the incubation mixtures and with the duration of incubation, the value of A*B will be independent of these parameters.(ABSTRACT TRUNCATED AT 250 WORDS) ER -